1. Field of the Invention
This invention relates to the treating and transporting of shale oil. More particularly, shale oil is thermally treated to reduce the arsenic content and to reduce the pour point, and the thus-treated oil is transported by pipeline and subsequently heated to produce coke and a liquid hydrocarbon distillate. Surprisingly, the liquid hydrocarbon distillate is hydroprocessed more easily than treated oil which has not been coked.
2. Statement of the Problem
The shale oil produced by conventional retorting processes has a number of characteristics which cause difficulties in transportation and/or catalytic hydroprocessing of the oil. Of these characteristics, one of the most bothersome is the high pour point of the retorted shale oil. "Pour point" is the temperature at which congelation or stoppage of flow is observed for a particular oil, and a high-pour-point oil is often difficult to handle at ambient temperature. There is no fixed relationship between the pour point and the viscosity of a given oil.
In the United States, most oil shale deposits are located in areas where the temperature during a good portion of the year is below 40.degree. F. (4.4.degree. C.), and often below freezing (32.degree. F., 0.degree. C.), while typical pour points of shale oils from existing retorting processes are in the range of 65.degree. to 85.degree. F. (18.degree. to 29.degree. C.) or more. Movement of the oil at temperatures only slightly above the pour point of an oil by ordinary fluid handling operations is difficult and commercially impractical; and at temperatures at or below the pour point, ordinary fluid handling of the oil is even more difficult. Therefore, transportation of shale oils, especially over relatively long distances by pipeline, is generally impractical unless expensive pour point depressants, expensive processing or heated pipelines are preliminarily employed.
Another detrimental characteristic of shale oil is that it frequently contains contaminants which tend to interfere with subsequent refining and catalytic processing operations such as hydrogenation. In some instances, these contaminants (soluble arsenic and iron in particular) may poison or inactivate catalysts used in such operations. Even if shale oil is employed directly as a fuel, the removal of such contaminants may be desirable from an environmental protection standpoint. Thus, it is desirable that arsenic, iron and other contaminants be removed or reduced to low concentrations in the shale oil before it is further processed or used as a fuel.
3. Description of the Prior Art
U.S. Pat. No. 3,284,336 and U.K. Pat. No. 995,106 disclose a process for reducing the pour point of shale oils by separating shale oil into light and heavy fractions, thermally treating the heavy fraction, and recombining both fractions. U.S. Pat. No. 3,738,931 discloses hydrovisbreaking shale oil, followed by hydrogenation of the vaporized visbroken oil and recombining the vapors with unvaporized oil to give a shale oil having a reduced pour point. U.S. Pat. No. 3,523,071 describes visbreaking and fractionation of a shale oil, with the higher boiling fraction of the visbroken shale oil combined with a portion of unvisbroken shale oil to give a low pour point product. U.S. Pat. No. 3,532,618 describes hydrovisbreaking shale oil to give a low pour point product. These references do not discuss contaminant removal by thermal treatment, or the effects of thermal treatment on the hydroprocessability of the oil.
Heretofore, arsenic has been removed from hydrocarbon charge stocks by contacting the charge stock with oxides of iron, cobalt or nickel and substantial amounts of water at a low temperature, as disclosed in U.S. Pat. No. 2,778,799. The oxide acts as an oxidizing agent which oxidizes the arsenic to a water-soluble arsenic oxide. The arsenic oxide is dissolved by the water and removed from the hydrocarbon. Also, arsenic has been removed from raw shale oil by contacting the shale oil in the absence of water with a catalyst, such as oxide or sulfide compounds of iron, cobalt or nickel at an elevated temperature under hydrogen pressure, see for instance U.S. Pat. Nos. 3,876,533; 3,933,624; 3,954,603; 4,003,829 and 4,051,022.
U.S. Pat. No. 4,029,571 discloses a method for thermally treating shale oil, either in the presence or the absence of hydrogen, to form an arsenic-containing precipitate suspended in the oil which must be subsequently separated. Although the method of this reference produces a treated oil having a reduced pour point and reduced levels of arsenic and selenium contaminants, the required step of precipitate removal (such as by centrifuging or filtering) is cumbersome, time-consuming and prone to mechanical difficulties.
In other uses, a thermal treating step has been employed to remove various metallic contaminants from petroleum hydrocarbons, as has been described in U.S. Pat. No. 2,910,434. This reference discloses removal of up to 26 various trace metals, but not arsenic, from a petroleum crude oil feed by non-catalytically reacting the feed with hydrogen in the presence of an inert packing material to form a treated oil of reduced metal content and a solid metal-containing residue. Although the packing may retain a portion of the residue, this reference requires that the treated oil and the remaining residue must be separated by means such as filtration and settling, which are time-consuming and prone to equipment failures. U.S. Pat. No. 3,947,347 discloses removal of the same metals from a hydrocarbon feed by contacting the feed with hydrogen and an inert packing material having a specified pore diameter range to deposit the contaminants on the inert material. U.S. Pat. No. B438,916 discloses demetallation (nickel, vanadium, iron, copper, zinc or sodium but not arsenic) of a residual petroleum fraction by contacting the oil with a refractory oxide in the absence of added hydrogen. These references do not concern arsenic removal or pour point reduction, nor do they recognize that the thermally treated oil is relatively difficult to hydroprocess when compared with the untreated shale oil. Further, they do not suggest a way to improve the hydrogen processability of the oil once it has been thermally treated.